Power transistor devices, such as power MOS transistors, include a drift region and a p-n junction between the drift region and a body region. The doping concentration of the drift region is lower than the doping concentration of the body region, so that a depletion region (space charge region) mainly expands in the drift region when the device blocks, which is when the p-n junction is reverse biased.
A length of the drift region in a current flow direction of the device and the doping concentration of the drift region mainly define the voltage blocking capability of the semiconductor device. In a unipolar device, such as a power MOSFET, the doping concentration of the drift region also defines the on-resistance of the device. The on-resistance is the electrical resistance of the semiconductor device in the on-state.
When the p-n junction is reverse biased dopant atoms are ionized on both sides of the p-n junction resulting in a space charge region that is associated with an electrical field. The integral of the magnitude of the field strength of the electrical field corresponds to the voltage that reverse biases the p-n junction, where the maximum of the electrical field is at the p-n junction. An avalanche breakthrough occurs when the maximum of the electrical field reaches a critical field strength that is dependent on the type of semiconductor material used to implement the drift region.
The doping concentration of the drift region may be increased without reducing the voltage blocking capability of the device when charges are provided in the drift region that may act as counter charges to ionized dopant atoms in the drift region when the p-n junction is reverse biased, which is when a depletion region expands in the drift region. Field electrodes or field plates may be provided in the drift region and dielectrically insulated from the drift region by a field electrode dielectric. These field electrodes may provide the required counter charges. These field electrodes may be electrically connected to a fixed electrical potential, such as gate or source potential in a MOS transistor.
The field electrode structure with the field electrode and the field electrode dielectric form a capacitive structure that, when the field electrode is connected to source potential, forms a part of the drain-source capacitance of the MOS transistor. The capacitive structure formed by the field electrode structure has a capacitance that is dependent on a load voltage between load terminals (drain and source terminals) of the transistor and decreases as the load voltage increases. In operation, the load voltage of the transistor increases when the transistor is switched from an on-state to an off-state. A reduced drain-source capacitance in the off-state reduces the capability of the transistor device to absorb voltage spikes. Those voltage spikes may, in particular, occur when the transistor switches from the on-state to the off-state and may result from parasitic devices and/or a load connected to the transistor device.